The present invention relates to a beam scanning apparatus, more specifically, to a laser beam scanning apparatus in which, for example, a scanning surface arranged with elements for recording image is scanned with a light beam emitted from a laser and information concerning the position at which the laser beam is applied on the scanning surface is precisely detected during image information recording, whereby the scanning on the scanning surface is realized with high precision.
Heretofore, there have been proposed various laser scanning apparatus in which a laser beam is deflected by a galvanometer or the like so as to be directed to a scanning surface on which elements for image recording are arranged, and image information such as characters or pictures, etc is recorded. This type of the apparatus is widely used for such apparatuses as a liquid crystal display, a laser marker and a laser trimmer that can write image information on the liquid crystal, metal plate or the like.
FIG. 1 schematically shows the main components of a galvanometer used as a deflector in conventional laser beam scanners to control the deflection of a laser beam.
In FIG. 1, the reference numeral 8 indicates a mirror fixed to a rotation shaft 9. An electric motor 10 rotates the shaft 9 at a constant speed. A spring 11, used to cause a rotating force in the reverse direction to the rotational direction of the shaft 9, is attached at one end to the shaft 9 and at the other end to the case 13.
A capacitance sensor 12, attached to the shaft 9 at its part to obtain information for the rotation of the shaft 9, detects the rotational angle of the shaft.
In FIG. 1, light beam incident on the mirror 8 from a laser (not shown) is deflected by oscillating the mirror 8 so as to scan the scanning surface. At this time, the rotation angle of the mirror is detected by the capacitance sensor 12 so as to control to drive the rotating state of the electric motor 10, and thus control the scanning state of the scanning surface.
In the conventional laser scanner shown in FIG. 1, however, there have arisen problems if scanning is attempted by rapid deflection of the laser beam.
(a) Rapid acceleration or deceleration causes a distortion of the rotation shaft. As a result, an error occurs between the rotation angle of the mirror and the rotation angle to be detected by the capacitance. Accordingly, it is hard to obtain accurate positional information of the applied position of the laser beam on the scanning surface. PA1 (b) If an attempt is made to lessen the moment of inertia of the rotating parts so as to improve the response characteristic, limitation will have to be imposed when attaching the capacitance sensor.